The general concept of utilizing electrically writeable and erasable phase change materials (i.e., materials which can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art, as is disclosed, for for example, in U.S. Pat. No. 3,271,591 to Ovshinsky, issued Sep. 6, 1966 and in U.S. Pat. No. 3,530,441 to Ovshinsky, issued Sep. 22, 1970, both of which are assigned to the same assignee as the present invention, and both disclosures of which are incorporated herein by reference (hereinafter the "Ovshinsky patents").
As disclosed in the Ovshinsky patents, such phase change materials can be changed between structural states of generally amorphous and generally crystalline local order or can be set between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The early materials described by the Ovshinsky patents could be switched between two detectable structural states of generally amorphous and generally crystalline local order to accommodate the storage and retrieval of single bits of encoded binary information or they could be set at intermediate detectable levels of local order over the entire spectrum between completely amorphous and completely crystalline states.
That is, the Ovshinsky patents describe that the electrical switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather that the detectable property, such as resistance, for example, can be set at any level over the whole range local order between the completely amorphous and the completely crystalline states. This property of varying local order provides a "gray scale" represented by the spectrum between the completely amorphous and the completely crystalline states. Such gray scale characteristic can be used as a substantially infinitely variable parameter such as, for example, a substantially infinitely variable resistance between maximum and minimum levels or it can be used to set incremental values of a selected parameter such as resistance, for example, in separately detectable steps between maximum and minimum levels.
The property of a variable parameter, such as resistance, adjustable over a gray scale range allows the application of these devices to neural network and artificial intelligence systems, for example, as described in the Ovshinsky patents. In another application of such devices as described in the Ovshinsky patents, the use of the element to set incremental and separately detectable levels of the parameter can accommodate the storage in a single memory element of multi-bit data.